Tiniest Ape Ever Discovered Hints at the Rise of the Monkeys

A tiny fossilized molar found nestled in the sweltering shrub land of Kenya’s Tugen Hills belonged to what may be the smallest species of ape yet discovered, according to a new study. The newly identified extinct species, Simiolus minutus, weighed only about 8 pounds, or slightly less than an average house cat.

Dwarfed by today’s gorillas, chimpanzees and orangutans, the miniature ape was possibly a casualty of natural selection, unable to compete with colobine monkeys that dined on the same leaves in trees some 12.5 million years ago.

“They were trying to do what colobines were doing, which was foolish because no one had that same equipment,” said James Rossie, a paleoprimatologist at Stonybrook University in New York, referring to the monkeys’ digestive abilities. “They brought a knife to a gunfight and then found out the knife was a plastic picnic knife.”

Dr. Rossie found the tooth in 2004 with a colleague, Andrew Hill from Yale University. Their finding, which was published online recently in the Journal of Human Evolution, provides insight into one aspect of an arms race between ancient apes and monkeys during the mid-to-late Miocene epoch some 6 to 14 million years ago.

The reason so many apes, including the small-body apes like Simiolus, disappeared is not clear. The prevailing hypotheses are that they died out because of competition from monkeys and environmental changes. Echoes from whatever happened during that period are still felt today as there are only about 20 species of apes, in contrast with more than 130 species of Old World monkeys in Africa and Asia. Habitat destruction by humans, though, is now the primary threat to species from both primate groups and the main reason their numbers have declined in recent times.

When Dr. Rossie found the molar, he and Dr. Hill realized that it looked similar to two teeth in a museum that had been collected in the 1970s and 1980s. Though they only had three teeth, Dr. Rossie said the teeth were different enough from anything else described that the pair knew they had a new species.

“It’s a bit like finding a single crashed Martian spacecraft out in the Arizona desert,” said Dr. Rossie. “You don’t need to find 12 to know what that means.”

The molar measured about 0.15 inches across. From the teeth, Dr. Rossie was able to extrapolate the size of the new ape’s jaw and body size, which were smaller than any living or known extinct species. The smallest living ape is the gibbon which weighs between 10 and 30 pounds.

By analyzing the shearing crests on the molar, they determined the species was at least a part-time folivore, or leaf-eater. Previously they had identified fossils of an early colobine monkey at the same site, which led them to suggest that the ape and monkeys competed against each other for food, providing a window into the larger fall of apes and rise of monkeys.

“Now we have a clear piece of the puzzle,” said Dr. Rossie.

Dr. Rossie said the finding, which took 14 years to publish, was a bittersweet tribute to his co-author and Ph.D adviser, Dr. Hill, who had developed leukemia and died in September 2015.

“To be honest, there was a sense in which we didn’t try very hard to finish it because working on it was a recurring reason to get together,” said Dr. Rossie.

“It’s sad to complete the last thing that I’ll ever do with him,” he said, “but at the same time, it gives me a great sense of pride to have done this with him.”